1. Field of the Invention
The present general inventive concept relates to an ink cartridge, and more particularly, to an ink cartridge including a residual ink sensing unit to sense an amount of ink remaining in an ink bag.
2. Description of the Related Art
Generally, an inkjet image forming apparatus includes a detachable ink cartridge to form an image by spraying ink to a printing medium. When the ink is exhausted, the ink cartridge is replaced by a new one. The ink cartridge includes a housing that houses the ink cartridge, a nozzle unit that sprays ink to the printing medium, an ink bag that contains ink to be supplied to the nozzle unit and flexibly shrinks according to ink consumption, and a meniscus that forms an ink path from the ink bag to the nozzle unit. When ink is sprayed from the nozzle unit, a negative pressure is generated, and ink contained in the ink bag is continuously provided to the nozzle unit due to the negative pressure.
When the ink contained in the ink bag is exhausted, the value of the negative pressure increases excessively, and consequently the nozzle unit and the meniscus can be damaged. To prevent damage to the nozzle unit and the meniscus and defective printing due to an unstable supply of ink, and to display the time for ink replacement, the ink cartridge includes a residual ink sensing unit.
FIG. 1 is a cross-sectional view illustrating a conventional residual ink sensing unit. Referring to FIG. 1, the residual ink sensing unit includes an ink bag 25, two terminals 24a and 24b, and an ink outlet 23. The ink bag 25 includes two flexible sidefilms 21 and 22. As an example, each sidefilm 21 and 22 is formed by three layers. An inner layer 19 of the ink bag 25 containing ink is easily attached to an intermediate layer 18, and manufactured with a thin film made of, for example, a polyethylene (PE) material in order to suppress chemical changes of toner. The intermediate layer 18 is formed by an aluminum film in order to protect the toner against the outside air and moisture. An outer layer 17 of the ink bag 25 is made of a material having a higher melting temperature than that of the material of the inner layer 19, and thus the outer layer 17 does not melt when the inner layer 19 is attached to the intermediate layer 18 by means of heat.
Both sidefilms 21 and 22 of the flexible ink bag 25 get closer to each other as the ink is consumed. At this time, a capacitance between the sidefilms 21 and 22 is changed, and a remaining amount of ink is sensed using a change of the capacitance. That is, electrodes 18a and 18b, formed by removing a part of the outer layer 17 and exposing a part of the intermediate aluminum layer 18, are connected to the terminals 24a and 24b, respectively, and a voltage value between the terminals 24a and 24b is measured. If the sidefilms 21 and 22 acting as capacitors and a resistance element (not shown) are appropriately connected to each other to form an RC circuit the change of the capacitance according to the approach of the sidefilms 21 and 22 is measured by a voltage difference.
However, a part of the inner layer 19 of the very thin ink bag 25 may come off. Due to this, insulation of the ink bag 25 is deteriorated, and thus, a functional characteristic of the sidefilms 21 and 22 as the capacitor is lowered, and reliability of data which is measured using the terminals 24a and 24b is decreased. Moreover, there is a problem in that the sensitivity of the residual ink sensing unit is lowered since the voltage difference is not proportional to the ink consumption.
Meanwhile, a hole is created in the sidefilms 21 and 22 of the ink bag 25 by high heat because the sidefilms 21 and 22 are very thin, and thus an ink storage capacity of the ink bag 25 can be deteriorated. Furthermore, to expose the electrodes 18a and 18b from the thin sidefilms 21 and 22, chemical materials or specific processes are needed, which make the manufacturing processes complicated.